The present invention relates to an image forming apparatus. More particularly, the present invention pertains to an image forming apparatus for forming image information, such as characters or figures, on the surface of an image carrying member by directing a modulated light beam emitted from a light source to the surface of the image carrying member having a predetermined shape, such as a photosensitive drum or a recording material, e.g., a silver salt film, through a light deflecting device, such as a rotating polygonal mirror, and by scanning the surface of the image carrying member using that light beam. Such an image forming apparatus can be suitably applied to, for example, a laser beam printer (LBP) or a process optical printer.
Conventional image forming apparatuses, such as laser beam printers, form image information by scanning the surface of an image carrying member by means of an optical scanning system using a light beam modulated by and emitted from a laser beam source.
Generally, in laser beam printers, the light modulated by and emitted from the light source is converged into a beam made up of parallel rays of light by means of a collimator lens, and then is made incident on a reflecting surface of a light deflecting device which may be a rotating polygonal mirror.
The light beam reflected by the reflecting surface of the light deflecting device is gathered by means of a scanning lens exhibiting f-.theta. characteristics, and is then directed to the surface to be scanned which is the image carrying member. The surface to be scanned is scanned using that light beam by rotating the light deflecting device by means of a motor or the like.
Recently there has been an increasing demand for an image forming apparatus capable of high density, i.e., high definition, recording of image information. To achieve high definition recording of the image information, in scanning the surface a light beam having a small spot size is generally required. However, a reduction in the spot size of the light beam results in a reduction of the depth of focus on the surface to be scanned, thus necessitating an optical image forming system (f-.theta. lens) which exhibits less curvature of the field.
To achieve a high definition or high quality image by reducing the light beam spot size on the surface to be scanned, a reduction in the F-number (FNo) of the optical image forming system, such as a f-.theta. lens, is desired.
However, a reduction in the FNo of the f-.theta. lens decreases the size of the light beam incident on the f-.theta. lens, and rapidly reduces the depth of focus. Hence, very high assembly accuracy of the surface to be scanned is required. This makes manufacture of the surface to be scanned very difficult.
In an optical scanning system provided with a means for performing tilt correction of the light deflecting device, such as a rotating polygonal mirror, it is very difficult to reduce the FNo of the f-.theta. lens. In the case of an optical scanning system having, for example, a scanning width corresponding to the A4 through A3 paper sizes, the FNo is generally between 20 and 30.
In the optical scanning apparatus which employs a laser beam as a light source, the diameter of the spot L and the depth of focus H on the surface to be scanned approximately have the following relationships: EQU L=k.lambda.F (1) EQU H=.+-.2.lambda.F.sup.2 ( 2)
where .lambda. is the wavelength of the light beam, F is the FNo of the optical system, and k is a constant (k.gtoreq.1.64) which represents the intensity of the light beam in the vicinity of the pupil of the f-.theta. lens. The spot diameter L is a value obtained when the intensity of light is reduced to 1/e.sup.2 of the maximum peak intensity.
Table 1 lists the spot diameter L and depth of focus H in the horizontal direction in which a light beam is moved. The values shown in Table 1 have been calculated using the wavelength .lambda.=0.78 .mu.m and the constant k=1.7.
TABLE 1 ______________________________________ F-Number of Spot Diameter Depth of Focus the Optical System (.mu.m) (mm) ______________________________________ 70 93 .+-.7.6 60 80 .+-.5.6 50 66 .+-.3.9 40 53 .+-.2.5 30 40 .+-.1.4 20 27 .+-.0.62 15 20 .+-.0.35 10 13 .+-.0.16 ______________________________________
As can be seen from the Table 1, when the FNo of the f-.theta. lens is, for example, about 15 and the spot diameter is 20 .mu.m or less, the depth of focus is .+-.0.35 mm or less.
Generally, the curvature of field on the surface to be scanned has different distribution characteristics in the horizontal (the meridional direction, indicated by a curve M in FIG. 1) and the vertical (the sagittal direction, indicated by a curve S in FIG. 1) directions, as shown in FIG. 1.
In an optical scanning apparatus, because the light beam diameter is large with respect to the vertical direction (in the direction perpendicular to the direction in which scanning is performed by the scanning means), the allowable degree of aberration is large. However, with respect to the horizontal direction (the direction in which scanning is performed by the scanning means), the light beam diameter is about one half of that in the vertical direction, and the allowable degree of aberration is thus small.
As can be seen from Table 1, when the spot diameter is about 20 .mu.m, the depth of focus is .+-.0.35 mm. Therefore, when the curvature of the field is .+-.0.35 mm, the aberration allowance of the surface to be scanned, e.g., the image carrying member, is zero in terms of the shape or arrangement thereof. As a result, if the spot diameter of the light beam is 20 .mu.m or less, a solution may not be available for the design of an optical scanning system using conventional techniques.
Accordingly, Japanese Laid-Open Patent No. 116603/1984 discloses an image forming apparatus which has a control system for correcting image formed position errors such that the position on the surface to be scanned at which the spot of a light beam is formed coincides with the focal point thereof.
However, the optical system of such an image forming apparatus is complicated, and hence the apparatus including the control system is very expensive.
In the case of an image forming apparatus having a post objective type optical scanning system, since the surface to be scanned of the image carrying member is cylindrical, the overall size of the image forming apparatus is large. Also, the construction of the conveying system is complicated.